DC offset compensation method and DC offset compensation device

ABSTRACT

A DC offset component that occurs in a quadrature modulation system, and that is contained in a modulated transmit signal, is compensated for with good accuracy. In a DC offset compensation method according to the present invention, a DC offset correction value obtained from the transmit signal is weighted in accordance with the signal level of an input signal which is transmit data input to the quadrature modulation system, and the DC offset component contained in the transmit signal is compensated for by using the thus weighted DC offset correction value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application and is based uponPCT/JP2005/00002, filed on Jan. 4, 2005.

TECHNICAL FIELD

The Substitute Specification contains no new matter.

The present invention relates generally to quadrature modulation inwhich a transmit signal is produced by modulating two quadraturecarriers in accordance with an input signal comprising an in-phasecomponent signal and a quadrature component signal, and moreparticularly to a DC offset compensation method and DC offsetcompensation device for compensating for a DC offset contained in thetransmit signal. More specifically, the invention relates to a DC offsetcompensation method and DC offset compensation device which performs theDC offset compensation by using a DC offset correction value obtainedfrom the transmit signal.

BACKGROUND ART

A quadrature modulation system, which produces a transmit signal bymodulating two quadrature carriers in accordance with an input signalcomprising an in-phase component signal and a quadrature componentsignal, can flexibly achieve a variety of modulation schemes and signalconstellations, and is therefore employed in many communicationapparatuses and electronic appliances.

FIG. 33 is a diagram schematically showing the configuration of such aquadrature modulation system. The input signal to the quadraturemodulation system 1, which is transmit data such as a complex basebandsignal, is made up of an in-phase component signal and a quadraturecomponent signal, and they are input to the quadrature modulation system1 via an I channel and a Q channel respectively corresponding to the twocarriers.

These input signal components are converted by D/A converters 13I and13Q, provided in the I and Q channels respectively, into analog signalsfor the respective channels. Then, the transmit signal is formed bymodulating the two carriers with the respective analog signals in aquadrature modulator 14, and the thus formed transmit signal is fed viaa power amplifier 15 to an antenna (not shown) for transmission.

In such a quadrature modulation system, when frequency-converting thetransmit signal, i.e., the complex baseband signal, by the analogquadrature modulator (QMOD), a DC offset may be added to the transmitsignal in analog device circuits in the quadrature modulation system asa whole, for example, in the analog device circuits between thequadrature modulator 14 and the digital/analog converters 13I and 13Q(hereinafter called the “D/A converters”), due to differences orvariations in the characteristics of multiplier circuits in the analogdomain.

This DC offset manifests itself as a carrier leakage (unwanted carrier)in the frequency-converted analog transmit signal, causing an adjacentchannel leakage and thus leading to a degradation of the transmit signalquality.

In one method practiced in the prior art to compensate for the DCoffset, a component inverse in polarity to the DC offset expected to beadded during the process between the D/A converters 13I and 13Q and thequadrature modulator 14 is added in advance to the transmit signalbefore input to the D/A converters 13I and 13Q, thereby canceling outthe DC offset.

To generate such a DC offset canceling signal, there is proposed amethod in which a portion of the quadrature-modulated transmit signal isfed back and the feedback signal is analyzed to measure and correct theDC offset (refer, for example, to patent document 1 listed below), andalso a method in which the transmit signal is subtracted from thefeedback signal to extract an error component and then the DC offset ismeasured and corrected.

FIG. 34 is a diagram showing a configuration example of a quadraturemodulation system in which the offset compensation is performed bygenerating a DC offset canceling signal. In this configuration example,the transmit signal is subtracted from the feedback signal to extract anerror component, and then the DC offset is measured and corrected.

For this purpose, a directional coupler 16 is inserted between the poweramplifier 15 and the antenna (not shown), and a portion of the transmitsignal is fed back through a monitor terminal of the directional coupler16. The transmit signal thus fed back is passed through a mixer 82, ananalog/digital converter (hereinafter called the “A/D converter”) 83,and a quadrature demodulator 84 to generate quadrature monitored signalsi and q, which are supplied to a DC offset correction value estimatingunit 20.

Then, based on these quadrature monitored signals i and q and theearlier described input signal components, the DC offset correctionvalue estimating unit 20 estimates DC offset correction values forcompensating for the DC offset for the in-phase and quadraturecomponents, respectively.

The DC offset correction values thus estimated are added by adders 12Iand 12Q respectively to the in-phase and quadrature input signalcomponents before input to the respective D/A converters.

An output signal of an oscillator 81 is input to the mixer 82 throughits local oscillator input terminal, and the transmit signal separatedby the directional coupler 16 is mixed with the local oscillator signalfor frequency conversion to produce an intermediate frequency signal.

The A/D converter 83 converts the intermediate frequency signal into adigital signal that is synchronized to a clock signal of a givenfrequency.

The quadrature demodulator 84 quadrature-demodulates the digital signalto produce the quadrature monitored signals i and q corresponding to theI and Q channels, respectively, that are in phase quadrature to eachother.

The DC offset correction value estimating unit 20 obtains offsetcomponents contained in the quadrature monitored signals i and q, forexample, by smoothing these signals in the complex plane. Likewise, theDC offset correction value estimating unit 20 obtains offset componentscontained in the in-phase and quadrature input signal components bysmoothing them in the complex plane.

Then, the DC offset correction value estimating unit 20 extracts onlythe offset component added in the quadrature modulation system 1 bysubtracting the in-phase signal component in the quadrature monitoredsignal i and the quadrature signal component in the quadranturemonitored signal q from the respective input components I and Q, andestimate the inverse offset obtained by inverting the sign of the offsetcomponent as the DC offset correction value.

Patent document 1: Japanese Unexamined Patent Publication No. H10-79692

Patent document 2: Japanese Unexamined Patent Publication No.2001-237723

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

As described above, the DC offset occurs due to differences orvariations in the characteristics of the multiplier circuits in theanalog domain in the quadrature modulation system as a whole.Accordingly, the amount of offset may vary according to the inputmodulating signal.

However, the above-described prior art DC offset compensation method,which generates the DC offset correction value by smoothing the feedbacksignal, has had the problem that a variation in the input signal is notreflected in the DC offset correction value and, therefore, compensationfor the DC offset cannot be performed with good accuracy.

In view of the above problem, it is an object of the present inventionto achieve quadrature modulation in which the DC offset contained in thetransmit signal can be compensated for with good accuracy.

Means for Solving the Problem

In quadrature modulation which produces a transmit signal by modulatingtwo quadrature carriers in accordance with an input signal comprising anin-phase component signal and a quadrature component signal, the presentinventors have discovered that the DC offset contained in the transmitsignal varies with the signal level of the input signal, and haveconceived of the invention based on this discovery.

In the present invention, as will be described in detail later withreference to drawings, a DC offset correction value is obtained from thetransmit signal and is weighted in accordance with the signal level ofthe input signal, and the DC offset is compensated for by using the thusweighted DC offset correction value.

In the present invention, the DC offset correction value is stored foreach signal level of the input signal. Then, the stored DC offsetcorrection value is retrieved based on the signal level of the inputsignal, and the DC offset is compensated for by using the thus retrievedDC offset correction value.

Further, in the present invention, an approximation equation isdetermined which is used to calculate from each signal level of theinput signal the DC offset correction value corresponding to that signallevel and, using this approximation equation, the DC offset correctionvalue that matches the signal level of the input signal is calculated,and the DC offset is compensated for by using the thus calculated DCoffset correction value.

In this way, by weighting or varying the DC offset correction value inaccordance with the signal level of the input signal, the effect thatthe variation of the input signal has on the offset is reflected in thecorrection value so that the DC offset can be compensated for with goodaccuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a first basic configuration of the presentinvention.

FIG. 2 is a graph showing the characteristics of a DC offset as afunction of input signal level.

FIG. 3 is a diagram showing a second basic configuration of the presentinvention.

FIG. 4 is a diagram showing a third basic configuration of the presentinvention.

FIG. 5 is a diagram schematically showing the configuration of aquadrature modulation system according to a first embodiment of thepresent invention.

FIG. 6 is a diagram (part 1) showing the configuration of a DC offsetcorrection value estimating unit shown in FIG. 5.

FIG. 7 is a diagram (part 2) showing the configuration of the DC offsetcorrection value estimating unit shown in FIG. 5.

FIG. 8A is a diagram showing the change of the input signal level withtime.

FIG. 8B is a diagram showing the change with time of a DC offsetcorrection value.

FIG. 8C is a diagram showing the DC offset correction value obtained byweighting the DC offset correction value of FIG. 8B in accordance withthe input signal level of FIG. 8A.

FIG. 9 is a diagram schematically showing the configuration of aquadrature modulation system according to a second embodiment of thepresent invention.

FIG. 10 is a diagram for explaining an averaging of the input signallevel.

FIG. 11 is a diagram schematically showing the configuration of aquadrature modulation system according to a third embodiment of thepresent invention.

FIG. 12 is a diagram schematically showing the configuration of aquadrature modulation system according to a fourth embodiment of thepresent invention.

FIG. 13 is a diagram schematically showing the configuration of aquadrature modulation system according to a fifth embodiment of thepresent invention.

FIG. 14 is a diagram showing the configuration of a weighting factorcalculating unit shown in FIG. 13.

FIG. 15 is a flowchart illustrating the operation for updating weightingfactor data stored in a weighting factor storing unit shown in FIG. 14.

FIG. 16 is a diagram schematically showing the configuration of aquadrature modulation system according to a sixth embodiment of thepresent invention.

FIG. 17 is a diagram showing the configuration of a weighting factorsetting unit shown in FIG. 16.

FIG. 18 is a flowchart illustrating the operation for updating theweighting factor data stored in the weighting factor storing unit shownin FIG. 17.

FIG. 19 is a diagram schematically showing the configuration of aquadrature modulation system according to a seventh embodiment of thepresent invention.

FIG. 20 is a diagram showing the configuration of the weighting factorsetting unit shown in FIG. 19.

FIG. 21 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with the frequency of the input signal.

FIG. 22 is a diagram schematically showing the configuration of aquadrature modulation system according to an eighth embodiment of thepresent invention.

FIG. 23 is a diagram showing the configuration of the weighting factorsetting unit shown in FIG. 22.

FIG. 24 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with ambient temperature.

FIG. 25 is a diagram schematically showing the configuration of aquadrature modulation system according to a ninth embodiment of thepresent invention.

FIG. 26 is a diagram showing the configuration of the weighting factorsetting unit shown in FIG. 25.

FIG. 27 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with the number of carriers.

FIG. 28 is a diagram schematically showing the configuration of aquadrature modulation system according to a 10th embodiment of thepresent invention.

FIG. 29 is a diagram schematically showing the configuration of aquadrature modulation system according to an 11th embodiment of thepresent invention.

FIG. 30A is a graph showing the relationship between DC offset amountand transmit signal level when there is no hysteresis.

FIG. 30B is a graph showing the relationship between DC offset amountand transmit signal level when there is a hysteresis.

FIG. 31 is a diagram schematically showing the configuration of aportion (17) of FIG. 29.

FIG. 32 is a diagram schematically showing the configuration of aquadrature modulation system according to a 12th embodiment of thepresent invention.

FIG. 33 is a diagram schematically showing a configuration example of aquadrature modulation system.

FIG. 34 is a diagram showing a configuration example of a quadraturemodulation system in which offset compensation is performed bygenerating a DC offset canceling signal.

DESCRIPTION OF THE REFERENCE NUMERALS

-   10. QUADRATURE MODULATING UNIT-   20. DC OFFSET CORRECTION VALUE ESTIMATING UNIT-   30. SIGNAL LEVEL DETECTING UNIT-   40. WEIGHTING FACTOR CALCULATING UNIT-   50. WEIGHTING UNIT-   60. DC OFFSET CORRECTION VALUE STORING UNIT-   70. DC OFFSET CORRECTION VALUE CALCULATING UNIT

BEST MODE FOR CARRYING OUT THE INVENTION

The basic configuration of the present invention will be describedbelow. FIG. 1 is a diagram showing a first basic configuration of thepresent invention. As shown, a DC offset compensation device accordingto the present invention comprises: a DC offset correction valueestimating unit 20 which estimates a DC offset correction value based ona transmit signal that is produced by a quadrature modulating unit 10 bymodulating two quadrature carriers in accordance with an input signalcomprising an in-phase component signal and a quadrature componentsignal; a signal level detecting unit 30 which detects the signal levelof the input signal; a weighting factor calculating unit 40 whichcalculates a weighting factor for weighting the DC offset correctionvalue in accordance with the signal level; and a weighting unit 50 whichassigns a weight to the DC offset correction value in accordance withthe weighting factor. The DC offset contained in the transmit signal iscompensated for by using the thus weighted DC offset correction value.

FIG. 2 shows the characteristics of the DC offset contained in themodulated transmit signal versus the level of the input signal to thequadrature modulation system that have been found experimentally by thepresent inventors. As shown, the DC offset contained in thequadrature-modulated transmit signal varies with the signal level of theinput signal with which the carriers are modulated. It is believed thatsuch a phenomenon occurs because variations in the input signal levelcause variations in the operating environment of the analog circuitssuch as the D/A converters 13I and 13Q previously shown in FIGS. 33 and34.

In view of this, in the basic configuration shown in FIG. 1, the DCoffset correction value for compensating for the DC offset is weightedin accordance with the signal level of the input signal so that the DCoffset varying with the signal level of the input signal can becompensated for with good accuracy.

FIG. 3 is a diagram showing a second basic configuration of the presentinvention. As shown, a DC offset compensation device according to thepresent invention comprises a signal level detecting unit 30 whichdetects the signal level of the input signal and a DC offset correctionvalue storing unit 60 which stores DC offset correction values forvarious input signal levels, and the DC offset contained in the transmitsignal is compensated for by using the DC offset correction valueretrieved based on the signal level of the input signal. In thisconfiguration also, by varying the DC offset correction value inaccordance with the signal level of the input signal, the DC offset canbe compensated for with good accuracy.

FIG. 4 is a diagram showing a third basic configuration of the presentinvention. As shown, a DC offset compensation device according to thepresent invention comprises a signal level detecting unit 30 whichdetects the signal level of the input signal and a DC offset correctionvalue calculating unit 70 which calculates the DC offset correctionvalue in accordance with the signal level by using a prescribedapproximation equation, and the DC offset contained in the transmitsignal is compensated for by using the DC offset correction valuecalculated in accordance with the signal level of the input signal. Inthis configuration also, by varying the DC offset correction value inaccordance with the signal level of the input signal, the DC offset canbe compensated for with good accuracy.

Embodiments of the present invention will be described below withreference to the accompanying drawings. FIG. 5 is a diagramschematically showing the configuration of a quadrature modulationsystem according to a first embodiment of the present invention.

The quadrature modulation system 1 receives transmit data, for example,a complex baseband signal or the like, as an input signal, and modulatestwo quadrature carriers with the input signal to produce a modulatedsignal for transmission.

The input signal is made up of an in-phase component signal and aquadrature component signal, which are input to the quadraturemodulation system 1 via an I channel and a Q channel respectivelycorresponding to the two carriers.

These input signal components are converted by D/A converters 13I and13Q, provided in the I and Q channels respectively, into analog signalsfor the respective channels. Then, a quadrature modulator 14 modulatesthe two carriers with the respective analog signals to produce thetransmit signal (modulated signal). This transmit signal is fed via apower amplifier 15 to an antenna (not shown) for transmission.

A directional coupler 16 is connected between the output of the poweramplifier 15 and the antenna (not shown), and the directional coupler 16separates a portion of the transmit signal through its monitor terminaland supplies it to a mixer 82.

In the mixer 82, the transmit signal supplied via the directionalcoupler 16 is mixed with a local oscillator signal from an oscillator 81for frequency conversion to produce an intermediate frequency signal,which is input to an A/D converter 83.

The A/D converter 83 converts the intermediate frequency signal into adigital signal that is synchronized to a clock signal of a givenfrequency (not shown).

A quadrature demodulator 84 quadrature-demodulates the digital signal toproduce quadrature monitored signals i and q corresponding to the I andQ channels, respectively, that are in phase quadrature to each other.

Then, based on this feedback signal, the DC offset correction valueestimating unit 20 estimates the DC offset correction value forcompensating for the DC offset.

To simplify the explanation of the DC offset correction value estimatingunit 20 given hereinafter, the section leading from the inputs of therespective D/A converters 13I and 13Q to the power amplifier 15 will bereferred to as the “forward line,” and the section leading from themonitor terminal of the directional coupler 16 to the input of the A/Dconverter 83 will be referred to as the “feedback line.”

FIG. 6 is a diagram schematically showing the configuration of the DCoffset correction value estimating unit 20.

As shown, the DC offset correction value estimating unit 20 comprises:an integrator 21-1 which smoothes the quadrature monitored signals i andq individually in the complex plane; a subtractor 22 one input of whichis connected to the output of the integrator 21-1 and the other input ofwhich is set to “0” which is the target value of the offset component tobe compensated for; a delay element 23-1 to which an output of thesubtractor 22 is supplied; a subtractor 24 one input of which isconnected to the output of the subtractor 22 and the other input ofwhich is connected to the output of the delay element 23-1; a conjugatecalculator 25 cascaded with the output of the subtractor 24; amultiplier 26 one input of which is connected to the output of theconjugate calculator 25; a multiplier 27 one input of which is connectedto the output of the subtractor 22 and the other input of which isconnected to the output of the multiplier 26; a multiplier 28 one inputof which is connected to the output of the multiplier 27 and the otherinput of which is supplied with a step size μ1; a delay element 23-2 viawhich the output of the multiplier 28 is connected to the other input ofthe multiplier 26; and a correction value calculating unit 29 whichcalculates an offset correction value vector from the output of themultiplier 28.

Operation of the thus configured DC offset correction value estimatingunit 20 will be described below.

The integrator 21-1 extracts the offset components contained in thequadrature monitored signals i and q by smoothing the respective signalsin the complex plane. The subtractor 22 obtains the deviationRx_(offset[n]) of each offset component relative to the target value “0”in order of time sequence n.

The delay element 23-1 and the subtractor 24 obtain the incrementδ_([n]) between the thus obtained Rx_(offset[n]) and Rx_(offset[n−1])(=Rx_(offset[n]−Rx) _(offset[n−1])) in order of time sequence n. Theconjugate calculator 25 obtains a conjugate increment δ_([n])′ which isthe conjugate of the increment δ_([n]) in the complex plane.

On the other hand, the delay element 23-2 stores an offset compensationvector CMP_([n]) and supplies the previously obtained offsetcompensation vector CMP_([n−1]) to the multiplier 26. The multiplier 26obtains the outer product u_([n]) of this previous offset compensationvector CMP_([n−1]) and the conjugate increment δ_([n]) ′ in order oftime sequence n.

Such an outer product u_([n]) is mathematically equivalent to the innerproduct of the offset compensation vector CMP_([n−1]) and the incrementδ_([n]); therefore, for simplicity, the outer product is hereinaftersimply referred to as the “inner product u_([n]),” and it is assumedthat the e^(j0) is set as the initial value u_([0).

The multipliers 27 and 28 sequentially update the offset compensationvector CMP_([n]) to the outer product shown by the equation (1) belowfor the inner product u_([n]), the deviation Rx_(offset[n]), and thestep size μ1, which is a preset scalar quantity.CMP _([n])=−μ1×Rx _(offset[n]) ×u _([n])  (1)

The correction value calculating unit 29 updates the offset correctionvalue vector Tx_(offset[n]) to the outer product (=Tx_(offset[n+1]))shown by the equation (2) below for the offset compensation vectorCMP_([n]) given by the multiplier 28 and the offset correction valuevector Tx_(offset[n]) which is set based on the offset compensationvector CMP_([n−1]) preceding the offset compensation vector CMP_([n]).Tx _(offset[n+1]) =Tx _(offset[n]) +CMP _([n])  (2)

The DC offset correction value estimating unit 20 supplies the thuscalculated DC offset correction value, i.e., the offset correction valuevector Tx_(offset[n]), to the adders 12I and 12Q via the weighting unit50, to be described later.

The adders 12I and 12Q add the in-phase and quadrature components of theoffset correction value vector Tx_(offset[n]) to the in-phase andquadrature components of the input signal, respectively, and supply theresults to the respective D/A converters 13I and 13Q.

Here, the offset compensation vector CMP_([n−1]) is a value with whichthe offset correction value vector Tx_(offset[n−1]) previously appliedto the forward line via the correction value calculating unit 29 is tobe updated.

Further, the increment δ_([n]) indicates the amount of change of thedeviation Rx_(offset[n]) that occurs in the feedback line when theoffset correction value vector Tx_(offset[n]) is applied to the forwardline instead of the previously applied offset correction value vectorTx_(offset[n−1]).

That is, the inner product u_([n]) of the offset compensation vectorCMP_([n−1]) and the increment δ_([n]) corresponds to the cosine value ofthe sum φ of the phase shifts in the forward and feedback lines, andthis value is updated as needed to a value that matches the deviation orvariation of the phase shift.

Accordingly, the thus generated offset correction value vectorTx_(offset[n]) is updated so as to minimize the expected value of theproduct of the deviation Rx_(offset[n]) and the inner product u_([n]),as shown by the above equations (1) and (2).

Further, the advantage is that the offset correction value vectorTx_(offset[n]) is maintained flexibly and stably at a value that matchesthe deviation or variation of the phase shift in the feedback line.

In the DC offset correction value estimating unit 20 shown in FIG. 6,“0” is given as the target value. However, as shown in FIGS. 5 and 7,since the target value is provided by an integrator 21-2 that receivesthe input signal and detects the DC component contained in the inputsignal, the DC offset correction value estimating unit 20 can also beapplied to a device that generates modulated waves containing residualcarrier signal components. In the embodiments hereinafter described,either the configuration shown in FIG. 6 or the one shown in FIG. 7 maybe employed for the DC offset correction value estimating unit 20.

Turning back to FIG. 5, the quadrature modulation system 1 includes asignal level detecting unit 30 which detects the signal level (forexample, power value or amplitude value) of the input signal, aweighting factor calculating unit 40 which calculates a weighting factorfor weighting the offset correction value vector Tx_(offset[n])(hereinafter simply called the “offset correction value”) in accordancewith the detected signal level, and a weighting unit 50 which assigns aweight to the DC offset correction value in accordance with theweighting factor.

As earlier described with reference to FIG. 2, the DC offset containedin the quadrature-modulated transmit signal varies with the signal levelof the input signal with which the carriers are modulated; therefore,when the DC offset correction value is weighted in accordance with thesignal level of the input signal by using the signal level detectingunit 30, weighting factor calculating unit 40, and weighting unit 50,the offset compensation can be performed using a DC offset correctionvalue that is closer to the DC offset contained in the transmit signalthan the prior art.

FIGS. 8A to 8C are diagrams for explaining how the DC offset correctionvalue is weighted according to the present invention: FIG. 8A is adiagram showing the change of the input signal level with time, FIG. 8Bis a diagram showing the change with time of the DC offset correctionvalue estimated by the DC offset correction value estimating unit 20,and FIG. 8C is a diagram showing the DC offset correction value obtainedby weighting the DC offset correction value of FIG. 8B in accordancewith the input signal level of FIG. 8A.

As described above, the signal level detecting unit 30 detects thesignal level which corresponds to the power value or amplitude value ofthe input signal. When the normal modulating signal is applied as thetransmit data, the detected signal level varies with time, as shown inFIG. 8A.

On the other hand, the DC offset correction value estimated by the DCoffset correction value estimating unit 20 as earlier described withreference to FIGS. 6 and 7 is obtained by integrating the quadraturemonitored signals i and q using the integrator 21-1 and thus averagingthese signals over an extended section; as a result, the signal valueobtained varies little over time as shown in FIG. 8B.

Therefore, the weighting factor calculating unit 40 shown in FIG. 5calculates the weighting factor in accordance with the signal leveldetected as shown in FIG. 8A, and the weighting unit 50 weights the DCoffset correction value shown in FIG. 8B in accordance with theweighting factor; that is, the DC offset correction value to be added tothe input signal by the adders 12I and 12Q is varied in accordance withthe input signal level, as shown in FIG. 8C, and the offset compensationis done using the thus weighted DC offset correction value as the finaloffset correction value.

Delay elements 11I and 11Q shown in FIG. 5 are provided to delay theinput signal by an amount of time required for the signal leveldetecting unit 30 to detect the input signal level and for the weightingfactor calculating unit 40 to calculate the weighting factor inaccordance with the signal level.

FIG. 9 is a diagram schematically showing the configuration of aquadrature modulation system according to a second embodiment of thepresent invention. While the weighting factor calculated by theweighting factor calculating unit 40 can be uniquely determined inaccordance with the signal level of the input signal, the DC offsetactually contained in the transmit signal may vary for the same inputsignal level because of unknown transient factors. Accordingly, in thepresent embodiment, averaging is performed to average the input signallevel by reference to which the weighting factor calculating unit 40calculates the weighting factor, and the DC offset correction value isweighted using the thus smoothed value.

For this purpose, the quadrature modulation system 1 includes, betweenthe signal level detecting unit 30 and the weighting factor calculatingunit 40, a signal level averaging unit 31 which averages the inputsignal level detected by the signal level detecting unit 30 over aprescribed interval (period) and outputs the average value.

When the signal level such as shown by a curve 90 in FIG. 10 is suppliedfrom the signal level detecting unit 30, the signal level averaging unit31 averages the signal level over a given interval and outputs itsaverage value 91 to the weighting factor calculating unit 40.

FIG. 11 is a diagram schematically showing the configuration of aquadrature modulation system according to a third embodiment of thepresent invention. In this embodiment, instead of performing averagingon the input signal level, the weighting factor being calculated by theweighting factor calculating unit 40 is averaged over a prescribedinterval, and the DC offset correction value is weighted in accordancewith the average value.

For this purpose, the quadrature modulation system 1 includes, betweenthe weighting factor calculating unit 40 and the weighting unit 50, aweighting factor averaging unit 32 which averages the weighting factorcalculated by the weighting factor calculating unit 40 over a prescribedinterval (period) and outputs the average value.

FIG. 12 is a diagram schematically showing the configuration of aquadrature modulation system according to a fourth embodiment of thepresent invention.

As can be seen from FIG. 2 previously shown, the variation of the DCoffset caused by the variation in the signal level of the input signaloccurs in both the amplitude and phase of the DC offset.

In the embodiments of the invention described herein, the weighting ofthe DC offset correction value may be performed with respect to eitherthe amplitude or the phase of the DC offset or to both.

Here, when performing the weighting of the DC offset correction valuewith respect to the phase and thus involving the rotation of the phase,the weighting factor calculating unit 40 may calculate complex weightingfactors Wi and Wq having in-phase and quadrature components I and Q,respectively, and a complex multiplication unit 51 as the weighting unitmay perform complex multiplications by multiplying the DC offsetcorrection value with the respective complex weighting factors, as shownin FIG. 12. The complex weighting factors Wi and Wq here may be given byequations (3) and (4) below.Wi=r×cos φ  (3)Wq=r×sin φ  (4)

Here, r is the weighting factor for the amplitude of the DC offset, andφ is the weighting factor for the phase.

FIG. 13 is a diagram schematically showing the configuration of aquadrature modulation system according to a fifth embodiment of thepresent invention, and FIG. 14 is a diagram showing the configuration ofthe weighting factor calculating unit 40 shown in FIG. 13. In thisembodiment, the weighting factor calculating unit 40 stores weightingfactor data for various input signal levels, and outputs from among thestored weighting factor data the weighting factor data that matches thesignal level output from the signal level detecting unit 30.

Further, in the present embodiment, the offset amount of the DC offsetcontained in the transmit signal is measured during the transmission ofthe signal, and the weighting factor stored in the weighting factorcalculating unit 40 is updated so as to minimize the offset amount.

For this purpose, the weighting factor calculating unit 40 comprises aweighting factor storing unit 41 for storing the weighting factor datafor various input signal levels, a weighting factor updating unit 42 forupdating the weighting factor data stored in the weighting factorstoring unit 41, and an offset amount measuring unit 43 for measuringthe offset amount of the DC offset contained in the transmit signal.

The weighting factor storing unit 41 is a dual port memory or amulti-port memory having at least a first address input (a) forreceiving as an input a read address associated with the signal level ofthe input signal, a first data port (b) for reading out the weightingfactor stored at the address received via the first address input andfor outputting the weighting factor to the weighting unit 50, a secondaddress input (c) for receiving as an input a write address specified bythe weighting factor updating unit 42, and a second data port (d) forreceiving as an input the weighting factor output from the weightingfactor updating unit 42 and for storing it at the address received viathe second address input.

The weighting factor updating unit 42 outputs a received-DC-offsetmeasuring instruction signal for instructing the offset amount measuringunit 43 to measure the amount of the DC offset contained in thequadrature monitored signals i and q received from the quadraturedemodulator 84, and receives the DC offset amount measured by the offsetamount measuring unit 43.

Further, the weighting factor updating unit 42 outputs to the DC offsetcorrection value estimating unit 20 a DC offset correction value updateinstruction signal for instructing the DC offset correction valueestimating unit 20 to estimate the DC offset correction value.

When the received-DC-offset measuring instruction signal is received,the offset amount measuring unit 43 measures the DC offset contained inthe quadrature monitored signals i and q received from the quadraturedemodulator 84, and returns the result to the weighting factor updatingunit 42.

The offset amount measuring unit 43 measures the integrated values ofthe quadrature monitored signals i and q, or the differences betweenthese integrated values and the integrated values of the input signals Iand Q, as the DC offset contained in the quadrature monitored signals iand q.

When the DC offset correction value update instruction signal isreceived, the DC offset correction value estimating unit 20 estimatesthe DC offset correction value, and outputs the newly updated DC offsetcorrection value to the weighting unit 50.

FIG. 15 is a flowchart illustrating the operation for updating theweighting factor data stored in the weighting factor storing unit 41shown in FIG. 14.

In step S10, the weighting factor updating unit 42 outputs the DC offsetcorrection value update instruction signal to the DC offset correctionvalue estimating unit 20. The DC offset correction value estimating unit20 that received this signal updates the DC offset correction value tothe latest value which is output.

In step S11, the weighting factor updating unit 42 gives permission toupdate the weighting factor data stored at a designated one of theaddresses in the weighting factor storing unit 41, i.e., the weightingfactor data for a designated one of the input signal levels (hereinaftercalled the “one-point weighting factor data”).

In step S12, the weighting factor updating unit 42 accesses thedesignated address, increments or decrements the one-point weightingfactor data by a prescribed small step, and writes it back to theweighting factor storing unit 41 to update the data.

To determine the update direction (increment or decrement) of theweighting factor data to be updated by the weighting factor updatingunit 42, the first update is done in either the incrementing ordecrementing direction, whichever is appropriate; then, the DC offsetamount measured in the next step is observed, and the update directionis automatically set to the direction that reduces the offset amount.

When an input signal having the corresponding signal level is applied,the updated one-point weighting factor data is read out from the readaddress associated with that signal level, and the data is used in theweighting unit 50 to weight the offset correction value. Here, updatingthe weighting factor data causes a variation in the DC offset containedin the transmit signal.

Therefore, in step S13, the weighting factor updating unit 42 outputsthe received-DC-offset measuring instruction signal to the offset amountmeasuring unit 43, and acquires the latest DC offset amount from theoffset amount measuring unit 43.

In step S14, it is determined whether the offset amount acquired in stepS13 has reached a minimum as a result of the updating of the one-pointweighting factor data. If the offset amount has reached a minimum, theupdating of the one-point weighting factor data is stopped (step S15),but if it has not yet reached a minimum, the process returns to step S12to update the one-point weighting factor data once again.

The determination as to whether the offset amount acquired in step S13has reached a minimum or not may be made, for example, by comparing theoffset amount measured in step S13 in the current loop with the offsetamount measured in the previous loop and by checking if the offsetamount that kept decreasing until the previous comparison has changedfrom decreasing to increasing, thus reaching a minimum.

Then, following steps S16 and S17, the process from steps S11 to S15 isrepeated until all the weighting factor data stored in the weightingfactor storing unit 41 are processed. In this way, the weighting factordata for every input signal level is updated.

FIG. 16 is a diagram schematically showing the configuration of aquadrature modulation system according to a sixth embodiment of thepresent invention, and FIG. 17 is a diagram showing the configuration ofthe weighting factor calculating unit 40 shown in FIG. 16. Thisembodiment is the same as the foregoing fifth embodiment in that theweighting factor calculating unit 40 stores weighting factor data forvarious input signal levels and outputs from among the stored weightingfactor data the weighting factor data that matches the signal leveloutput from the signal level detecting unit 30, but differs in that theweighting factor data is set in advance by using a training signalbefore signal transmission of the quadrature modulation system 1.

For this purpose, the weighting factor calculating unit 40 comprises thepreviously described weighting factor storing unit 41, a weightingfactor setting unit 44 for setting the weighting factor data to bestored in the weighting factor storing unit 41, and the previouslydescribed offset amount measuring unit 43.

The weighting factor setting unit 44 outputs a received-DC-offsetmeasuring instruction signal for instructing the offset amount measuringunit 43 to measure the amount of the DC offset contained in thequadrature monitored signals i and q received from the quadraturedemodulator 84, and receives the DC offset amount measured by the offsetamount measuring unit 43.

Further, the weighting factor setting unit 44 supplies, to atransmitting device (not shown) transmitting an input signal (transmitdata) to the quadrature modulation system 1, an unmodulating setupsignal for causing the transmitting device to transmit a training signalwhich is an unmodulating signal with a constant signal level instead ofthe usual transmit data which is a modulating signal having a varyingsignal level.

FIG. 18 is a flowchart illustrating the operation for updating theweighting factor data stored in the weighting factor storing unit 41shown in FIG. 17.

In step S20, the quadrature modulation system 1 receives as an input theusual transmit data generated by the transmitting device (not shown).

Then, in step S21, the weighting factor setting unit 44 outputs thereceived-DC-offset measuring instruction signal to the offset amountmeasuring unit 43, and acquires the amount X of the DC offset containedin the modulating signal.

Next, in step S22, the weighting factor setting unit 44 sends theunmodulating setup signal to the transmitting device (not shown). Inresponse, the transmitting device (not shown) transmits to thequadrature modulation system 1 the training signal which is aconstant-level unmodulating signal instead of the usual transmit datawhich is a modulating signal.

In step S23, the weighting factor setting unit 44 outputs thereceived-DC-offset measuring instruction signal to the offset amountmeasuring unit 43, and acquires the DC offset amount Y when the trainingsignal is input.

In step S24, the weighting factor setting unit 44 calculates thedifference between the DC offset amount Y measured in step S23 and theDC offset amount X measured in step S21, and stores the difference asthe weighting factor data corresponding to the signal level of thetraining signal in the weighting factor storing unit 41.

Then, following steps S25 and S26, the process from steps S23 to S24 isrepeated for each signal level while varying the signal level of thetraining signal, and the weighting factor data for each signal level isstored in the weighting factor storing unit 41.

FIG. 19 is a diagram schematically showing the configuration of aquadrature modulation system according to a seventh embodiment of thepresent invention, and FIG. 20 is a diagram showing the configuration ofthe weighting factor calculating unit 40 shown in FIG. 19. In thisembodiment, the weighting factor calculating unit 40 adjusts theweighting factor for the DC offset correction value in accordance withthe frequency of the input signal.

For this purpose, the weighting factor calculating unit 40 comprises, asshown in FIG. 20, a plurality of weighting factor storing units 41 eachfor storing the weighting factor data associated with each signal levelof the input signal for a plurality of frequencies of the input signal.Each weighting factor storing unit 41 is constructed from a plurality ofstoring means such as memories for storing the weighting factor data forthe respective frequencies of the input signal.

The weighting factor calculating unit 40 further comprises: a switchingcontrol unit 45 for receiving frequency information indicating thefrequency of the input signal from the transmitting device (not shown)transmitting the transmit data to the quadrature modulation system 1,and for performing switching so as to select the memory that forms theweighting factor storing unit 41 associated with the frequencyinformation; an address switching unit 46 for performing switching toconnect the level signal of the signal level detecting unit 30 as theread address to the address input of the memory of the weighting factorstoring unit 41 selected by the switching control unit 45; and a dataswitching unit 47 for performing switching to connect the data output ofthe selected memory to the weighting unit 50.

FIG. 21 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with the frequency of the input signal.

In step S30, the switching control unit 45 in the weighting factorcalculating unit 40 of FIG. 20 receives frequency information indicatingthe frequency of the input signal from the transmitting device (notshown). If there is a change in frequency, in step S31 the switchingcontrol unit 45 in FIG. 20 selects the memory that forms the weightingfactor storing unit 41 associated with the received frequencyinformation, and connects the selected memory to the level signal outputof the signal level detecting unit 30 and an input of the weighting unit50 in FIG. 19 via the address switching unit 46 and the data switchingunit 47, respectively.

FIG. 22 is a diagram schematically showing the configuration of aquadrature modulation system according to an eighth embodiment of thepresent invention, and FIG. 23 is a diagram showing the configuration ofthe weighting factor calculating unit 40 shown in FIG. 22. In thisembodiment, the weighting factor calculating unit 40 adjusts theweighting factor for the DC offset correction value in accordance withthe ambient temperature at which quadrature modulation is performed bythe quadrature modulation system 1.

For this purpose, the weighting factor calculating unit 40 comprises, asshown in FIG. 23, a plurality of weighting factor storing units 41 eachfor storing the weighting factor data associated with each signal levelof the input signal for a plurality of ambient temperatures at which thequadrature modulation is performed. Each weighting factor storing unit41 is constructed from a plurality of storing means such as memories forstoring the weighting factor data for the respective ambienttemperatures.

The weighting factor calculating unit 40 further comprises: a switchingcontrol unit 45 for receiving ambient temperature information of thequadrature modulation system 1 from an external temperature sensor (notshown), and for performing switching so as to select the memory thatforms the weighting factor storing unit 41 associated with the ambienttemperature; an address switching unit 46 for performing switching toconnect the level signal of the signal level detecting unit 30 as theread address to the address input of the memory of the weighting factorstoring unit 41 selected by the switching control unit 45; and a dataswitching unit 47 for performing switching to connect the data output ofthe selected memory to the weighting unit 50.

FIG. 24 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with the ambient temperature.

In step S40, the switching control unit 45 in the weighting factorcalculating unit 40 of FIG. 23 receives ambient temperature informationindicating the ambient temperature from the temperature sensor (notshown). If there is a change in ambient temperature, in step S41 theswitching control unit 45 in FIG. 23 selects the memory that forms theweighting factor storing unit 41 associated with the received ambienttemperature information, and connects the selected memory to the levelsignal output of the signal level detecting unit 30 and an input of theweighting unit 50 in FIG. 22 via the address switching unit 46 and thedata switching unit 47, respectively.

FIG. 25 is a diagram schematically showing the configuration of aquadrature modulation system according to a ninth embodiment of thepresent invention, and FIG. 26 is a diagram showing the configuration ofthe weighting factor calculating unit 40 shown in FIG. 25. In thisembodiment, the weighting factor calculating unit 40 adjusts theweighting factor for the DC offset correction value in accordance withthe number of carriers forming the input signal.

For this purpose, the weighting factor calculating unit 40 comprises, asshown in FIG. 26, a plurality of weighting factor storing units 41 eachfor storing the weighting factor data associated with each signal levelof the input signal for different numbers of carriers forming the inputsignal. Each weighting factor storing unit 41 stores the weightingfactor data for the different numbers of carriers forming the inputsignal.

The weighting factor calculating unit 40 further comprises: a switchingcontrol unit 45 for receiving carrier count information indicating thenumber of carriers forming the input signal from the transmitting device(not shown) transmitting the transmit data to the quadrature modulationsystem 1, and for performing switching so as to select the memory thatforms the weighting factor storing unit 41 associated with the carriercount information; an address switching unit 46 for performing switchingto connect the level signal of the signal level detecting unit 30 as theread address to the address input of the memory that forms the weightingfactor storing unit 41 selected by the switching control unit 45; and adata switching unit 47 for performing switching to connect the dataoutput of the selected memory to the weighting unit 50.

FIG. 27 is a flowchart illustrating the operation for adjusting theweighting factor in accordance with the number of carriers forming theinput signal.

In step S50, the switching control unit 45 in the weighting factorcalculating unit 40 of FIG. 26 receives carrier count informationindicating the number of carriers forming the input signal from thetransmitting device (not shown). If there is a change in the number ofcarriers, in step S51 the switching control unit 45 selects theweighting factor storing unit 41 associated with the received carriercount information, and connects the selected unit to the level signal ofthe signal level detecting unit 30 and an input of the weighting unit 50via the address switching unit 46 and the data switching unit 47,respectively.

FIG. 28 is a diagram schematically showing the configuration of aquadrature modulation system according to a 10th embodiment of thepresent invention. According to this embodiment, the DC offset amountcan be compensated for by using a simple configuration in the case ofthe quadrature modulation system in which the DC offset amount issubstantially proportional to the signal level of the input signal.

For this purpose, the quadrature modulation system 1 includes: amultiplier 46 as a weighting factor calculating unit for multiplying thesignal level detected by the signal level detecting unit 30 by aconstant 1/α and thereby calculating the weighting factor proportionalto the signal level; and multipliers 52I and 52Q for multiplying the DCoffset correction value output from the DC offset correction valueestimating unit 20 by the thus calculated weighting factor.

FIG. 29 is a diagram schematically showing the configuration of aquadrature modulation system according to an 11th embodiment of thepresent invention. In this embodiment, DC offset correction values arestored, one for each different signal level of the input signal. Then, acorresponding one of the stored DC offset correction values is retrievedbased on the signal level of the input signal, and the DC offset iscompensated for by using the thus retrieved DC offset correction value.

For this purpose, the quadrature modulation system 1 includes a DCoffset correction value storing unit 60 for storing the DC offsetcorrection values for various signal levels of the input signal. The DCoffset correction value storing unit 60 may be constructed from a lookuptable (LUT) or the like. The DC offset correction value corresponding tothe input signal level detected by the signal level detecting unit 30 isretrieved from the DC offset correction value storing unit 60 by usingthe signal level as the read address, and the DC offset correction valuethus retrieved is supplied to the adders 12I and 12Q.

The DC offset correction value that the DC offset correction valueestimating unit 20 estimates from the transmit signal may be storeddirectly as the DC offset correction value in the DC offset correctionvalue storing unit 60.

If there is a variation in the DC offset contained in the transmitsignal generated from the input signal of the same signal level, the DCoffset correction value stored in the DC offset correction value storingunit 60 may be updated in such a manner that the stored DC offsetcorrection value is incrementally brought closer to the DC offsetcorrection value output from the DC offset correction value estimatingunit 20, rather than updating the stored DC offset correction value tothe output value itself each time the DC offset correction value isoutput from the DC offset correction value estimating unit 20.

For this purpose, the quadrature modulation system 1 includes, as shownin FIG. 29, a DC offset correction value updating unit 61 for updatingthe DC offset correction value stored in the DC offset correction valuestoring unit 60 in such a manner that the stored DC offset correctionvalue incrementally approaches the current DC offset correction valueestimated by the DC offset correction value estimating unit 20.

Then, based on the currently stored DC offset correction valueTxm_([n+]) and the DC offset correction value Tx_(offset[n]) output fromthe DC offset correction value estimating unit 20, the DC offsetcorrection value updating unit 61 calculates the updated DC offsetcorrection value Txm_([n+1]) to be stored in the DC offset correctionvalue storing unit 60, for example, in accordance with the followingequation (5).Txm _([n+1]) =Txm _([n])+μ2×Tx _(offset[n])  (5)

In this way, the DC offset correction value may be stored in the DCoffset correction value storing unit 60 while incrementally updating thevalue. Here, μ2 is a constant that defines the size of the incrementalstep.

Now consider the case where there is a hysteresis in the DC offsetcontained in the transmit signal, as will be described with reference toFIGS. 30A and 30B. FIG. 30A is a graph showing the relationship betweenthe DC offset amount and the transmit signal level when there is nohysteresis, and FIG. 30B is a graph showing the relationship between theDC offset amount and the transmit signal level when there is ahysteresis. When there is such a hysteresis, the offset amount of the DCoffset contained in the transmit signal does not show the same value forthe same transmit signal level when the signal level increases as whenit decreases.

Accordingly, to compensate for such an offset amount, the DC offsetcorrection value storing unit 60 shown in FIG. 29 must be mademultidimensional so that a plurality of offset correction values can bestored for the same input signal in accordance with the amount by whichthe input signal level changes with time.

FIG. 31 is a diagram showing a configuration example of the input signalreceiving stage 17 of FIG. 29 when the DC offset correction valuestoring unit 60 of the quadrature modulation system shown in FIG. 29 ismade multidimensional in order to compensate for the DC offset amounthaving a hysteresis characteristic.

This configuration includes a signal level change calculating unit 67which comprises a delay element 62 for delaying the output of the signallevel detecting unit 30 and a subtractor 63 one input of which isconnected to the output of the delay element 62 and the other input ofwhich is directly connected to the output of the signal level detectingunit 30. The signal level change calculating unit 67 calculates theamount of change of the input signal level with time by calculating thedifference between the current input signal level and the precedinginput signal level.

Here, the memory space of the DC offset correction value storing unit 60is, for example, made two-dimensional. Further, the DC offset correctionvalue storing unit 60 is constructed as a two-dimensional lookup tablewith the output of the signal level detecting unit 30 as the firstdimension read address and the output of the subtractor 63 as the seconddimension read address. By employing such a configuration, when there isa hysteresis in the DC offset amount, the DC offset correction valuestoring unit 60 can store DC offset correction values that not onlydiffer in accordance with the signal level of the input signal, but alsodiffer in accordance with the amount of change of the input signal levelwith time. As a result, data that not only matches the signal level ofthe input signal but also matches the amount of change calculated by thesignal level change calculating unit can be retrieved from among thestored DC offset correction values.

Further, in the above configuration, when writing a DC offset correctionvalue to the DC offset correction value storing unit 60, the output ofthe signal level detecting unit 30 may be used as the first dimensionwrite address and the output of the subtractor 63 as the seconddimension write address. By using such write addresses, the DC offsetcorrection value estimated by the DC offset correction value estimatingunit 20 is written to a different address if either the signal level ofthe input signal or the amount of change of the signal level with timeis different.

In this configuration, when an input signal having a signal levelcorresponding to the first dimension address and an amount of changewith time corresponding to the second dimension address isquadrature-modulated, the DC offset correction value output from the DCoffset correction value estimating unit 20 can be stored in the DCoffset correction value storing unit 60 at an address expressed by thefirst and second dimension addresses. Delay elements 64 and 65 areprovided to delay the first and second dimension addresses by an amountof time that elapses from the moment that the input signal isquadrature-modulated until the DC offset correction value is output fromthe DC offset correction value estimating unit 20.

FIG. 32 is a diagram schematically showing the configuration of aquadrature modulation system according to a 12th embodiment of thepresent invention. In this embodiment, an approximation equation isdetermined which is used to calculate from each signal level of theinput signal the DC offset correction value corresponding to that signallevel and, using this approximation equation, the DC offset correctionvalue that matches the signal level of the input signal is calculated,and the DC offset is compensated for by using the thus calculated DCoffset correction value.

For this purpose, the quadrature modulation system 1 includes: a DCoffset correction value calculating unit 70 for calculating the DCoffset correction value in accordance with each signal level of theinput signal by using a prescribed approximation equation; a parameterstoring unit 71 for storing parameters for defining the prescribedapproximation equation; and a parameter calculating unit 73 forcalculating the parameters defining the approximation equation from theDC offset correction value output from the DC offset correction valueestimating unit 20.

The approximation equation may, for example, be a polynomial with thesignal level of the input signal as the variable, and the parameters maybe the coefficients by which the terms contained in the polynomial arerespectively multiplied. In this case, the DC offset correction valuecalculating unit 70 calculates the DC offset correction value inaccordance with the signal level of the input signal by using thepolynomial.

The parameter calculating unit 73 calculates the parameters using, forexample, a least square method, based on the respective signal levels ofa plurality of input signals and on the DC offset correction values thatthe DC offset correction value estimating unit 20 estimates from therespective transmit signals when the input signals of the respectivesignal levels are respectively quadrature-modulated.

While the DC offset compensation method and DC offset compensationdevice according to the present invention can be advantageously used tocompensate for a DC offset that is added to a transmit signal in ananalog quadrature modulation system used in high-speed communicationssuch as IMT2000, the present invention is not limited to this particularapplication, but can be widely used for quadrature modulation systemsthat use two carriers in phase quadrature and produce a transmit signalby modulating the two carriers in accordance with an input signalcomprising an in-phase component signal and a quadrature componentsignal.

1. A DC offset compensation method for compensating for a DC offsetcomponent contained in a transmit signal produced by modulating twoquadrature carriers in accordance with an input signal comprising anin-phase component signal and a quadrature component signal, saidcompensation being performed using a DC offset correction value obtainedfrom said transmit signal, wherein said DC offset correction value isweighted in accordance with a signal level of said input signal.
 2. TheDC offset compensation method as claimed in claim 1, wherein said DCoffset correction value is weighted in accordance with an average valueobtained by averaging the signal level of said input signal over aprescribed period.
 3. The DC offset compensation method as claimed inclaim 1, wherein said DC offset correction value is weighted inaccordance with an average value obtained by averaging a weightingfactor that matches the signal level of said input signal over aprescribed period.
 4. The DC offset compensation method as claimed inclaim 1, wherein said DC offset correction value is weighted for anamplitude component and/or a phase component of said DC offsetcomponent.
 5. The DC offset compensation method as claimed in claim 1,wherein a weighting factor to be applied to said DC offset correctionvalue is stored, and said DC offset correction value is weighed inaccordance with said stored weighting factor.
 6. The DC offsetcompensation method as claimed in claim 5, wherein an offset amount ismeasured for said DC offset component contained in said transmit signal,and said stored weighting factor is updated so as to minimize saidoffset amount.
 7. The DC offset compensation method as claimed in claim1, wherein a weighting factor to be applied to said DC offset correctionvalue is set and stored in advance for each signal level of said inputsignal, and said DC offset correction value is weighted in accordancewith said set weighting factor.
 8. The DC offset compensation method asclaimed in claim 7, wherein an offset amount is measured for said DCoffset component contained in said transmit signal modulated whenarbitrary transmit data is input as said input signal, an offset amountis measured for said DC offset component contained in said transmitsignal modulated when a training signal of a signal level correspondingto said each signal level is input as said input signal, and theweighting factor to be applied to said DC offset correction value iscalculated for each signal level of said input signal, based on thedifference between the offset amount for said arbitrary transmit dataand the offset amount for said training signal of said each signallevel.
 9. The DC offset compensation method as claimed in claim 1,wherein each weighting factor to be applied to said DC offset correctionvalue is adjusted in accordance with the frequency of said input signal.10. The DC offset compensation method as claimed in claim 1, whereineach weighting factor to be applied to said DC offset correction valueis adjusted in accordance with an ambient temperature at which themodulation is performed.
 11. The DC offset compensation method asclaimed in claim 1, wherein each weighting factor to be applied to saidDC offset correction value is adjusted in accordance with the number ofcarriers forming said input signal.
 12. The DC offset compensationmethod as claimed in claim 1, wherein said DC offset correction value isweighted in proportion to the signal level of said input signal.
 13. ADC offset compensation method used in quadrature modulation whichproduces a transmit signal by quadrature-modulating two quadraturecarriers in accordance with an input signal comprising an in-phasecomponent signal and a quadrature component signal, for compensating fora DC offset component contained in said transmit signal by using a DCoffset correction value obtained from said transmit signal, comprising:storing said DC offset correction value for each signal level of saidinput signal; retrieving said stored DC offset correction value based onsaid signal level; and compensating for said DC offset component byusing said retrieved DC offset correction value.
 14. The DC offsetcompensation method as claimed in claim 13, wherein said DC offsetcorrection value is stored in a correction value storage table which isaccessed by specifying an address associated with the signal level ofsaid input signal.
 15. The DC offset compensation method as claimed inclaim 13, wherein said DC offset correction value is estimated from saidtransmit signal corresponding to said input signal having said signallevel, and said DC offset correction value stored for said each signallevel is updated based on said estimated DC offset correction value. 16.The DC offset compensation method as claimed in claim 13, wherein saidDC offset correction value is stored in association with the signallevel of said input signal and the amount of change of the signal levelof said input signal with time, said stored DC offset correction valueis retrieved based on the signal level of said input signal and theamount of change of the signal level of said input signal with time, andsaid DC offset component is compensated for by using said retrieved DCoffset correction value.
 17. The DC offset compensation method asclaimed in claim 16, wherein said DC offset correction value isestimated based on said signal level and on said transmit signalcorresponding to said input signal exhibiting said amount of change, andsaid estimated DC offset correction value is stored in association withsaid signal level and said amount of change.
 18. The DC offsetcompensation method as claimed in claim 16, wherein said DC offsetcorrection value is estimated based on said signal level and on saidtransmit signal corresponding to said input signal exhibiting saidamount of change, and said DC offset correction value stored inassociation with said signal level and said amount of change is updatedbased on said estimated DC offset correction value.
 19. A DC offsetcompensation method used in quadrature modulation which produces atransmit signal by modulating two quadrature carriers in accordance withan input signal comprising an in-phase component signal and a quadraturecomponent signal, for compensating for a DC offset component containedin said transmit signal by using a DC offset correction value obtainedfrom said transmit signal, comprising: determining an approximationequation for calculating from each signal level of said input signalsaid DC offset correction value corresponding to said each signal level;calculating said DC offset correction value that matches the signallevel of said input signal by using said approximation equation; andcompensating for said DC offset component by using said calculated DCoffset correction value.
 20. The DC offset compensation method asclaimed in claim 19, wherein said DC offset correction valuecorresponding to said each signal level is estimated based on saidtransmit signal corresponding to said input signal having said signallevel, parameters for defining said approximation equation are obtainedfrom said DC offset correction value estimated for said each signallevel, and based on said approximation equation and said parameters,said DC offset correction value for compensating for said DC offsetcomponent is calculated in accordance with the signal level of saidinput signal.
 21. A DC offset compensation device used in quadraturemodulation which produces a transmit signal by modulating two quadraturecarriers in accordance with an input signal comprising an in-phasecomponent signal and a quadrature component signal, for compensating fora DC offset component contained in said transmit signal by using a DCoffset correction value obtained from said transmit signal, comprising:a DC offset estimating unit which estimates said DC offset correctionvalue based on said transmit signal; a signal level detecting unit whichdetects signal level of said input signal; a weighting factorcalculating unit which calculates, based on said detected signal level,a weighting factor to be applied to said DC offset correction value; anda weighting unit which weights said DC offset correction value inaccordance with said weighting factor.
 22. The DC offset compensationdevice as claimed in claim 21, further comprising a signal levelaveraging unit which calculates an average value by averaging the signallevel of said input signal over a prescribed period, wherein saidweighting factor calculating unit calculates said weighting factor inaccordance with said average value.
 23. The DC offset compensationdevice as claimed in claim 21, further comprising a weighting factoraveraging unit which calculates an average value by averaging saidweighting factor over a prescribed period, wherein said weighting unitweights said DC offset correction value in accordance with said averagevalue.
 24. The DC offset compensation device as claimed in claim 21,wherein said weighting factor is a weighting factor to be applied tosaid DC offset correction value with respect to an amplitude componentand/or a phase component of said DC offset component.
 25. The DC offsetcompensation device as claimed in claim 21, further comprising aweighting factor storing unit which stores each weighting factor to beapplied to said DC offset correction value, wherein said weighting unitweights said DC offset correction value in accordance with said storedweighting factor.
 26. The DC offset compensation device as claimed inclaim 25, further comprising: an offset amount measuring unit whichmeasures from said transmit signal an offset amount for the DC offsetcomponent contained in said transmit signal; and a weighting factorupdating unit which minimizes said measured offset amount by updatingsaid weighting factor stored for said DC offset correction value. 27.The DC offset compensation device as claimed in claim 25, furthercomprising: a weighting factor setting unit which sets each weightingfactor to be applied to said DC offset correction value for each signallevel of said input signal, wherein said weighting factor storing unitstores said each weighting factor preset by said weighting factorsetting unit.
 28. The DC offset compensation device as claimed in claim27, further comprising: an offset amount measuring unit which measuresfrom said transmit signal an offset amount for the DC offset componentcontained in said transmit signal, wherein said weighting factorcalculating unit calculates said each weighting factor based on thedifference between the offset amount of the DC offset component measuredin said transmit signal modulated when arbitrary transmit data is inputas said input signal and the offset amount of the DC offset componentmeasured in said transmit signal modulated when a training signal of asignal level corresponding to said each signal level is input as saidinput signal.
 29. The DC offset compensation device as claimed in claim21, wherein said weighting factor calculating unit adjusts eachweighting factor to be applied to said DC offset correction value inaccordance with the frequency of said input signal.
 30. The DC offsetcompensation device as claimed in claim 21, wherein said weightingfactor calculating unit adjusts each weighting factor to be applied tosaid DC offset correction value in accordance with an ambienttemperature at which said quadrature modulation is performed.
 31. The DCoffset compensation device as claimed in claim 21, wherein saidweighting factor calculating unit adjusts each weighting factor to beapplied to said DC offset correction value in accordance with the numberof carriers forming said input signal.
 32. The DC offset compensationdevice as claimed in claim 21, wherein said weighting factor calculatingunit calculates said weighting factor in proportion to the signal levelof said input signal.
 33. A DC offset compensation device used inquadrature modulation which produces a transmit signal by modulating twoquadrature carriers in accordance with an input signal comprising anin-phase component signal and a quadrature component signal, forcompensating for a DC offset component contained in said transmit signalby using a DC offset correction value obtained from said transmitsignal, comprising: a signal level detecting unit which detects signallevel of said input signal; a DC offset correction value storing unitwhich stores said DC offset correction value for each signal level ofsaid input signal, wherein said stored DC offset correction value isretrieved based on said detected signal level, and said DC offsetcomponent is compensated for by using said retrieved DC offsetcorrection value.
 34. The DC offset compensation device as claimed inclaim 33, further comprising: a DC offset correction value estimatingunit which estimates said DC offset correction value based on saidtransmit signal; and a DC offset correction value updating unit which,based on said estimated DC offset correction value, updates said DCoffset correction value stored in said DC offset correction valuestoring unit.
 35. The DC offset compensation device as claimed in claim33, further comprising: a signal level change calculating unit whichcalculates the amount of change of the signal level of said input signalwith time, wherein said DC offset correction value storing unit storessaid DC offset correction value in association with the signal level ofsaid input signal and the amount of change of the signal level of saidinput signal with time, and said stored DC offset correction value isretrieved based on the signal level of said input signal and the amountof change calculated by said signal level change calculating unit, andsaid DC offset component is compensated for by using said retrieved DCoffset correction value.
 36. The DC offset compensation device asclaimed in claim 35, further comprising: a DC offset correction valueestimating unit which estimates said DC offset correction value based onsaid transmit signal, wherein said DC offset correction value isestimated by said DC offset correction value estimating unit based onsaid signal level and on said transmit signal corresponding to saidinput signal exhibiting said amount of change, and said estimated DCoffset correction value is stored in association with said signal leveland said amount of change.
 37. The DC offset compensation device asclaimed in claim 35, further comprising: a DC offset correction valueestimating unit which estimates said DC offset correction value fromsaid transmit signal; and a DC offset correction value updating unitwhich, based on said estimated DC offset correction value, updates saidDC offset correction value stored in said DC offset correction valuestoring unit.
 38. A DC offset compensation device used in quadraturemodulation which produces a transmit signal by modulating two quadraturecarriers in accordance with an input signal comprising an in-phasecomponent signal and a quadrature component signal, for compensating fora DC offset component contained in said transmit signal by using a DCoffset correction value obtained from said transmit signal, comprising:a signal level detecting unit which detects signal level of said inputsignal; a DC offset correction value calculating unit which calculatessaid DC offset correction value in accordance with the signal level ofsaid input signal by using a prescribed approximation equation, whereinsaid DC offset component is compensated for by using said calculated DCoffset correction value.
 39. The DC offset compensation device asclaimed in claim 38, further comprising: a DC offset correction valueestimating unit which estimates said DC offset correction value based onsaid transmit signal; a parameter calculating unit which calculatesparameters for defining said approximation equation from said DC offsetcorrection value estimated based on said transmit signal correspondingto said input signal having said signal level; and a parameter storingunit which stores said calculated parameters, wherein based on saidprescribed approximation equation and said parameters, said DC offsetcorrection value calculating unit calculates said DC offset correctionvalue for compensating for said DC offset component.